FEEDER SYSTEMS.
Fig. 83.
There are two general schemes of direct current feeding in common use. One of these is shown in [Fig. 83]. Here the trolley wire is continuous and is fed into at different points. The long feeders supplying the more distant portion of the road are larger than those supplying the trolley near by, so as to maintain as nearly as is feasible the same potential the entire length of the line. With such a system of feeding, in order to maintain absolutely the same voltage at all points, it would be necessary to have just one trolley feeder and that feeding into the extreme end of the line farthest from the power station and further to make the resistance per 1,000 ft. of trolley and feeder the same as the resistance per 1,000 ft. of the track return circuit. The plan shown in [Fig. 83] evidently does not fully carry out these rather impracticable requirements but is in the nature of a compromise, giving a higher potential near the power station than at distant points but nevertheless much more even potential than if the heaviest feeders were feeding into the trolley near the power house.
The other plan, shown in [Fig. 84], divides the trolley wire into sections and feeds each section through a separate feeder which is calculated of such size as to maintain the same voltage on all the sections with the ordinary load.
In calculating a feeder system a certain probable load is assumed at certain points along the line. This load will manifestly depend on the size and number of cars in operation, grades and many local conditions.
Fig. 84.
| Drop in rail section | 3.1 Volts | 2.1 Volts | 1.05 Volts |
| Total drop in rail | 3.1” | 5.2” | 6.25” |
| Drop in trolley | 20.5” | 20.5” | 20.5” |
| Drop in feeder | 36.4” | 34.3” | 33.25” |
| Resistance feeder | .728 Ohms | .686 Ohms | .665 Ohms |
| Feet per ohm | 7253 | 23000 | 39700 |
| Size of wire | No. 1 | 250,000 C. M. | 420,000 C. M. |
The following example will show the method pursued. The figures resulting from the calculations are placed immediately below the sections to which they refer in [Fig. 84]. The rails are assumed to be 70 pound to the yard. These have a resistance of about .018 ohms per mile. Adding one-sixth for additional resistance of bonds gives .021 and since the track is composed of two rails the resistance of the track will be one-half of this or .0105 ohms per mile.
The maximum drop in any section occurs when the car is farthest from the power house. Each car is assumed to take 50 amperes and the feeders are to be so designed as to allow a 10 per cent or 60 volts drop.
The current in the two miles of track nearest the power house is 150 amperes, in the next section 100 amperes, and in the last section 50 amperes. The drop in each section is as shown. The drop in the trolley which is 00 wire is, in each section, 20.5 volts. Subtracting from 60 volts the drop in the return circuit and trolley, gives the allowable drop in the feeder.
The resistance of each feeder can be calculated, since the current in each one is 50 amperes. The first feeder is one mile long, the second 3 miles and the third 5 miles, and with these figures the feet per ohm can be computed. The size of wire may be obtained by reference to a table of copper wire resistances.